1. Field of the Invention
This invention relates to imaging members, such as lithographic printing plates, which contain a cross-linked hydrophilic coating. In particular, this invention relates to a method of improving the non-image performance of printing plates having an imaging layer containing a crosslinking agent having epoxy groups and ionic groups.
2. Background Information
Lithographic printing is based upon the immiscibility of oil and water, wherein an oily material or ink is preferentially retained by an imaged area and the water or fountain solution is preferentially retained by the non-imaged areas. When a suitably prepared negative working printing plate is moistened with water and ink is then applied the background or non-imaged areas retain the water and repel the ink while the imaged areas accept the ink and repel the water. The reverse holds true for positive working plates, in which the background is imaged. The ink is then transferred to the surface of a suitable substrate, such as cloth, paper or metal, thereby reproducing the image.
Very common lithographic printing plates include a metal or polymer support having thereon an imaging layer sensitive to visible or UV light. Both positive and negative-working printing plates can be prepared in this fashion. Upon exposure, and perhaps post-exposure heating, either imaged or non-imaged areas are removed using wet processing chemistries.
Thermally sensitive printing plates are less common, yet represent a steadily growing market. Currently, most of these plates utilize similar materials and similar imaging mechanisms as UV-imageable plates. For example, a thermal acid generator might be used in lieu of a photoacid generator and the same series of preheat and development steps might be employed. The main advantage of these digital plates is that the thermal imaging process is rapid and inexpensive compared to the analog process involving the creation of a mask and blanket UV exposure. While these plates can be imaged using lasers and digital information, they require wet processing using alkaline developer solutions.
Plates which do not require a separate wet processing step using a developer (“process-less” plates) are also of great interest in the art, particularly non-ablation plates. One approach toward non-process, non-ablation printing plates involves the use of “switchable polymers.” These polymers will undergo thermally driven chemical reactions in which highly polar moieties are either created or destroyed under imaging conditions. This results in the storage of the imaging data as hydrophilic and hydrophobic regions of a continuous polymer surface. In addition to not needing wet processing, such plates have the advantage of not needing any type of material collection devices which ablation-based plates require. Also, unlike ablation plates, a switchable polymer plate in its ideal form would consist of one layer and can be manufactured on a single pass through a coating machine.
Although a number of switchable polymer-based printing plates are known, there remain technical barriers toward the utilization of this technology in commercially feasible products. Three difficulties commonly experienced in the design of switchable polymer-based plates are physical wear of the plates, and the related problems of background scumming and blanket toning. In particular, the problems of scumming, also known as “toning”, and blanket toning typically result if ink-rejecting areas of the plate are not sufficiently polar. The uptake of ink in undesired areas of the plate results in the consequent undesirable transfer of ink to the final prints. Scumming may occur in both negative-working plates, in nonimaged areas, and positive plates, in imaged areas. The related problem of blanket toning refers to the buildup of ink in the background areas of the printing press blanket cylinder. Excessive blanket toning results in the necessity of periodically stopping a press run to manually clean the ink from the blanket. This can have a negative impact on the productivity of a printing process.
In switchable polymer-based printing plates, a major challenge lies in the creation of a synthetic polymer surface that has both adequate physical toughness and resistance to toning. In general, surfaces that reject ink well tend to be very highly hydrophilic and thus when exposed to an aqueous fountain solution they may be dissolved and lose adhesion to the support substrate. Alternatively, they may swell and become prone to abrasion and wear. It can be expected, then, that many of the synthetic polymer surfaces that are most resistant to toning will also have inherently inadequate physical properties for use in long-run printing plates. It is not uncommon that approaches to improve a switchable polymer plate's scumming behavior by increasing the hydrophilicity of the imageable layer will result in a consequent decrease in the wear resistance of the plate. Similarly, efforts to improve the physical toughness of a plate can result in an increase in scumming propensity.
Coatings cross-linked with compounds containing one or more epoxy groups (herein referred to as “epoxy compounds”) have been used in “process-less” printing plates which can be thermally imaged. For example, WO 01/39985 describes imaging members that require no wet processing after imaging. Other thermal imaging compositions containing epoxy resins are described in EP 1 075 942 and in JP 2001-31875.
None of the prior art discloses printing plates containing a crosslinking agent having epoxy groups and ionic groups. Accordingly, the present invention is directed to imaging members, preferably printing plates, containing an imaging layer comprising as crosslinking agents which contain a plurality of epoxy groups and one or more ionic group. These cross-linking agents improve the background performance of printing plates, preferably of printing plates which contain polymers comprising ionic groups. Moreover, the cross-linking agents can be incorporated in high concentrations into coatings without adversely affecting the ability to keep non-printing areas clean.